Sulfur hexafluoride (SF6) is a hypervalent, inorganic, colorless, odorless, non-toxic and non-flammable gas compound that is poorly soluble in water. It has a density of 6.1 g/L. Some 75% of the 8,000 tons of SF6 produced per year is used as a gaseous dielectric medium in the electrical industry; as an inert gas for the casting of magnesium; and as an inert filling for insulated glazing windows. SF6 plasma is also used in the semiconductor industry as an etchant. Because SF6 is relatively slowly absorbed by the bloodstream, it is used to provide a long-term plug of a retinal hole in retinal detachment repair operations. SF6 is employed as a contrast agent for ultrasound imaging of peripheral vein to enhance the visibility of blood vessels to ultrasound. This application has been utilized to examine the vascularity of tumors. Sulfur hexafluoride is also used as a fluorophilic gaseous diluent for ammonia in the efficient reaction with fluorine to produce nitrogen trifluoride.
New fluoride recovery methods for management of uranium fluorides in the nuclear industry could produce large quantities of uranium-free sulfur tetrafluoride. An efficient process for production of sulfur hexafluoride from sulfur tetrafluoride is required to further increase the commercial value of this approach to nuclear waste management. There is incentive to develop lower cost of producing sulfur hexafluoride.
Meanwhile, currently patented processes involve either direct fluorination of sulfur in an electrolytic reactor with byproducts including SF4, and S2F10; or use of oxygen to oxidize sulfur in sulfur tetrafluoride with production of sulfur dioxide as byproduct. Also, the reaction of xenon tetrafluoride with sulfur tetrafluoride to produce sulfur hexafluoride has been reported in the literature. However, this process is cost prohibitive as an industrial process. Evaluation of the feasibility of efficient alternative process for production of sulfur hexafluoride from sulfur tetrafluoride has involved a review of the thermodynamic data, such as the enthalpy (ΔH), Gibbs free energy (ΔG), and equilibrium constant (log K) of the theoretical process by using the HSC Chemistry 7.0 software. The comparative data, (ΔH) and log k, for oxidation of S(IV) to S(VI) by oxygen (O2), fluorine (F2), xenon tetrafluoride (XeF4), and cobalt trifluoride (CoF3) are shown in Tables 1 and 2.
The results suggest that cobalt trifluoride, a strong fluorinating agent, could produce a basis for new reactor concept for the efficient oxidation of sulfur tetrafluoride. Indeed, CoF3 fluorinates sulfur tetrafluoride in a gas-solid heterogonous reaction to produce sulfur hexafluoride. This process does not produce undesired side products like S2F10 or SO2, but result in the desired sulfur hexafluoride. The simple reactor for this process avoids the cost of energy for the electrolytic method.
Cobalt trifluoride is a common fluorinating agent for commercial production of many perfluorinated organic compounds form saturated and unsaturated hydrocarbons. As shown in Equation 1, in those reactions cobalt trifluoride is reduced to difluoride:CoF3+R—H→CoF2+R—F+HF  Eqn. 1
Passing a steady flow of elemental fluorine through a CoF3 bed produces an efficient CoF2/CoF3 oxidation-reduction system. Therefore, a process for CoF3 catalyzed fluorination of sulfur tetrafluoride; to produce sulfur hexafluoride is described in this patent.